The invention relates to an optical printer, comprising a light source, a light switching mask with light switching elements and an optical imaging system which is arranged between the light switching mask and a photo-sensitive record carrier in order to transfer a light dot raster generated in the light switching mask to the record carrier.
An optical printer of this kind is known, for example, from U.S. Pat. No. 4,278,981 and its principle is shown in FIG. 1. For the light switching mask, use is made of a row of magneto-optical light switching elements (adjacent elements being regularly, e.g. equally, distanced from one another) whose construction and operation are described in DE-OS No. 26 06 596. Other optical printing heads comprise rows of light switching elements manufactured by way of the liquid crystal technique. A further technique utilizes ceramic electro-optical materials for the construction of a light switching array.
Optical printing heads are used, for example, in electro-photographic printers for the line-wise exposure of an optically sensitive record carrier or intermediate carrier on which, subsequently, an optical image is formed by means of, for example, a photographic method or, in the case of electro-photography, by means of an electro-photographic method. Notably, electro-photographic printers are becoming increasingly more important for printing systems and office systems for the high-quality printing of text or graphic on normal paper.
Basically, an as high as possible density of electronically individually switchable light dots is desired for optical printing heads in order to increase the image quality. For the application in electro-photographic printers, the aim is for a density of at least 10 light dots per millimeter. However, in so-called laser printing heads, a resolution of up to 16 light dots per millimeter is presently achievable.
When solid state light switching masks are used, for example, as disclosed in U.S. Pat. No. 4,278,981, the desirable dot density can be achieved, but the manufacturing technique imposes a limit as regards the absolute length of a light switching row. For example, the described magneto-optical light switching masks are manufactured by means of a photolithographic masking technique. They can have a length of at the most a few centimeters. For example, light switching masks have been realized according to the magneto-optical principle which masks comprise up to 512 switching elements integrated on one carrier with a density of 16 switching elements per millimeter. Dot densities of 20 switching elements/mm can also be achieved without problems by means of the present techniques.
For the exposure of a line of DIN A4 format sheet in an optical printer, modular-like linking of several of such light switching masks comprising one row of light switching elements is then required. For example, an optical printer has been proposed in which several light switching rows are adjacently arranged, each light switching row being imaged on the record carrier by means of its own objective.
It is a drawback of such a construction of an optical printing head that the imaging distance between the object plane of the light switching mask and the image plane of the record carrier is comparatively large, because aperture and focal length are limited when use is made of separate objectives for the imaging of a line having a length of several centimeters. Typical imaging distances for the imaging of approximately 500 light dots in a 16 dot/mm raster are from 15 to 20 cm. Moreover, the diameter of the objectives must also be comparatively large in order to achieve a high aperture ratio so that as much light as possible of the object dots is intercepted. The comparatively high cost of such separate objectives is also disadvantageous.
The required imaging volume can in principle be reduced by a division into shorter light switching masks with a correspondingly larger number of imaging objectives. This is because the focal length of objectives may be chosen to be smaller when the object field to be imaged is smaller. However, it is a drawback of such a solution that the mounting costs are increased because it involves the use of a larger number of light switching masks and objectives which must all be exactly positioned with respect to one another in order to produce a gap-free and straight line of light dots on the record carrier in the image plane.